A rotorcraft is piloted while monitoring numerous instruments on the instrument panel, most of which instruments are representative of the operation of the power plant of the rotorcraft. For physical reasons, there are numerous limits that the pilot needs to take into account at all times during a flight. These various limits depend in general on the stage of the flight and on external conditions.
Most presently-constructed twin-engined rotorcraft are fitted with two free turbine engines for driving the main lift and propulsion rotor. Power is then taken from a low-pressure stage of each free turbine, which stage is mechanically independent of the compressor assembly and of the high-pressure stage of the turbine engine. Each free turbine of the turbine engines has a speed of rotation lying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a stepdown gearbox is needed in the connection to the main rotor whose own speed of rotation lies substantially in the range 200 rpm to 400 rpm: this is the main transmission gearbox.
Thermal limits on the turbine engine and limits on the torque that the main transmission gearbox can accept enable three power ratings to be defined for normal use of the turbine engine:                takeoff rating, that can be used for 5 to 10 minutes, corresponding to a level of torque on the gearbox and a level of heating in the turbine engine that can be accepted for a limited length of time without significant degradation: this is the maximum takeoff power (PMD);        the maximum continuous power rating during which capabilities are not exceeded at any time, this applies both to the capabilities of the transmission gearbox and to the capabilities that result from maximum heating that can be accepted continuously by the high-pressure blades of the first stage of the turbine: this is maximum continuous power (PMC); and        maximum transient rating, optionally capped by regulation: reference is then made to maximum transient power (PMT).        
There also exist supercontingency excess power ratings that are used only when one of the two turbine engines breaks down:                the supercontingency rating during which the capabilities of the transmission gearbox in its inlet stages and the thermal capabilities of the turbine engine are used to the maximum: this is referred to as super-emergency power (PSU), it can be used during 30 consecutive seconds at most, and only three times in any one flight. If the PSU is used, then the turbine engine must be removed and overhauled;        the supercontingency rating during which the capabilities of the inlet stages of the transmission gearbox and the capabilities of the turbine engine are used to a large extent: this is referred to as maximum emergency power (PMU) that can be used for 2 minutes following PSU or for 2 minutes and 30 seconds consecutively at most; and        the supercontingency rating during which the capabilities of the inlet stages of the transmission gearbox and the thermal capabilities of the turbine engine are used without damage: this is referred to as intermediate emergency power (PIU) and it can be used for 30 minutes or continuously for the remainder of the flight after the turbine engine has broken down.        
Under such conditions, the engine manufacturer determines, by calculation or by testing, curves representing the power available from a turbine engine as a function of altitude and temperature, and does so for each of the above-defined ratings. Similarly, the manufacturer determines the lifetime of the turbine engine and the minimum power that is guaranteed for each rating, where the guaranteed minimum power corresponds to the power that the turbine engine will deliver at the end of its service life, and where such a turbine engine is referred to below for convenience as an “old” turbine engine.
In order to verify that the turbine engine is operating properly, it is therefore appropriate to carry out a health check to ensure that the turbine engine is delivering performance better than or equal to the performance of an old turbine engine.
Two surveillance parameters are particularly important for checking the performance of a turbine engine.
Since the turbine engine has a high-pressure turbine located upstream from a free turbine, one surveillance parameter is the turbine entry temperature (TET) of the gas at the entry to the high-pressure turbine.
The blades of the high-pressure turbine of the turbine engine are subjected to centrifugal force and to the temperature TET. Above a certain level, the material from which the blades are made becomes subjected to creep, thereby having the consequence of lengthening the blades. Thus, the blades begin to touch the casing of the high-pressure turbine, and they are thus degraded. The temperature TET is thus linked directly to degradation of the turbine engine.
Nevertheless, it is very difficult to measure the temperature TET because of its relatively non-uniform nature, so the first surveillance parameter is preferably the temperature of the gas at the entry to the free turbine, known as T45 to the person skilled in the art. This temperature is a good image of the temperature TET, and consequently it is representative of turbine engine degradation.
Another surveillance parameter relates to the power delivered by the turbine engine, referred to as W by the person skilled in the art, or in equivalent manner the torque from the turbine engine, given that the power and the torque of an engine are closely linked. Nevertheless, the speed of rotation of the turbine engine gas generator, referred to as Ng by the person skilled in the art, is proportional to the power delivered by the turbine engine, so the second surveillance parameter actually used can also be the speed of rotation of the gas generator.
Below, verifying the state of health of a turbine engine consists either:                in controlling the turbine engine to reach a given value for the first surveillance parameter and then verifying that the value of the second surveillance parameter is less than or equal to the value that the second surveillance parameter would take in an old turbine engine under the same conditions; or else        controlling the turbine engine to reach a given value for the second surveillance parameter and then verifying that the value of the first surveillance parameter is less than or equal to the value that the first surveillance parameter would take in an old turbine engine under the same conditions.        
The health check must be carried out rigorously since if it is performed poorly, i.e. if the above-mentioned verifications do not give satisfactory results, that will have a non-negligible impact on potential immobilization of the rotorcraft and on its maintenance costs.
In this configuration, it is appropriate firstly to determine whether the poor result of the health check is the consequence of malfunction of the power plant rather than of the turbine engine. Secondly, it might then be necessary to remove the turbine engine so that an operator, e.g. the manufacturer of the turbine engine, can verify the degradation in its performance on a test bench and then replace the defective elements.
It can thus be understood that it is essential to carry out the health check with the greatest possible care so as to avoid immobilizing a rotorcraft without good reason. Unfortunately, it is extremely difficult to carry out a health check under good conditions on a twin-engined rotorcraft.
For such an aircraft, a first solution consists in carrying out a health check during a cruising flight, which presents the advantage of the check being carried out during a stage of flight without disturbances and thus with a turbine engine that is well stabilized.
Nevertheless, the power developed by turbine engines during such a flight is well below the reference powers, i.e. the maximum takeoff power (PMD), for example. Unfortunately, it is found that a health check is not accurate unless the power developed by the turbine engine being checked is close to its reference power.
Furthermore, on a twin-engined rotorcraft, it is appropriate to ensure that each engine is capable of developing the minimum power guaranteed during supercontingency ratings. Consequently, it is preferable to perform the health check at a rating that is as close as possible, in terms of power developed, to such supercontingency ratings. As a result, health checks are preferably implemented at a power close to the maximum takeoff power PMD, but that is not compatible with cruising flight.
A second solution consists in carrying out a health check while cruising at high speed, by increasing the power developed by the turbine engines so as to approach PMD, for example. Although effective, that solution leads to complaints from the passengers of a rotorcraft who are disturbed by the vibration generated in the cabin as a result of flying conditions.
A third solution consists merely in increasing the power developed by the turbine engine being checked. Although tempting, that solution suffers from drawbacks.
Since the rotorcraft is twin-engined, its turbine engines will then be unmatched in terms of power. Consequently, modern turbine-engine computers detect a loss of power. Under such conditions, a red alarm is activated by the computers to inform the pilot that a aircraft must imperatively be landed. Furthermore, such detection leads to contingency ratings being engaged.
Such a situation has indeed been tried by pilots seeking to carry out a health check on a turbine engine, but it remains forbidden by the authorities responsible for aviation if there are passengers present on the rotorcraft. It will readily be understood that no risks must be taken, and flying with an active alarm of the most serious type together with a reduced safety margin, cannot be accepted since this configuration leads to a significant and harmful increase in the workload on the pilot of the rotorcraft.
As a result, owners of a twin-engined rotorcraft have had until now only one solution for checking the state of health of the turbine engines of a rotorcraft, namely carrying out a special technical flight dedicated to checking health. The impact of doing this on the maintenance cost of the rotorcraft is not negligible insofar as the manufacturer of the turbine engine generally requires health checks to be performed every 25 hours. Similarly, each technical flight takes the place of a paid-for flight, and that ends up constituting a major cost for the owner of a rotorcraft.